Web-shaped plastic foils of the type described above are previously known, for example, from EP-A-O 353 496 and EP-A-O 353 991. The prior art web-shaped plastic foils are produced from a particulate starting material of propylene homopolymer or ethylene-propylene copolymer and between 50 and 80 weight per cent filler, and are intended to be used as packaging materials for configurationally stable, liquid-tight food packages. Similar web-shaped plastic foils are also known from GB-A-1 554 143.
According to the above-described prior art technology, see for example GB-A-1 554 143, the web-shaped plastic foil is produced from particulate starting materials of polymer and filler, by an extrusion process comprising three mutually discrete process steps which each requires its own individual process equipment and which it has not hitherto been possible to integrate into a single continuous process cycle.
In the prior art extrusion process, a pulverulent polymer is fed, in a first process stage, together with such components as stabilisers, into a first extrusion plant which produces stabilised polymer granules by extrusion in a known manner. In a second process stage, the stabilised polymer granules are fed from the first process stage together with particulate filler into a second extrusion plant which produces stabilised polymer granules including filler, by extrusion in a known manner. The stabilised polymer granules containing filler from the second process stage are finally fed, in a third process stage, into a third extrusion plant which produces web-shaped plastic foil by extrusion of the polymer material through a flat gap die aperture. The extruded plastic foil is then calendered to the desired foil thickness with the aid of calender rollers disposed in association with the discharge end of the third extrusion plant.